ASK THE EXPERTS: Sunspace Sizing

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I am a student at Hocking College in Nelsonville, Ohio. For our energy class, I am assigned the task of designing and building a sunspace for a mobile, energy-efficient building (16 feet wide × 16 feet long × 10 feet high). How do I correctly size this sunspace and make it usable as living space?

Kelley Barstow • via email

It sounds like a great project! The sizing of the sunspace depends on what your objectives are. The most ambitious goal would be to heat the cabin using only the sunspace, and even include solar-heat storage. You’d need to make sure that the cabin is well-insulated and tight. This is a very tall order.

A less ambitious goal is to keep the sunspace environment warm enough for plants all day, and warm enough for people during the day and for a few hours after sunset. Within these constraints, you’d seek to generate a bit of space heating for the cabin. This is the way a lot of conventional sunspaces are sized.

For example, let’s take a look at a less ambitious goal, and try to size a low-mass sunspace so that, on a sunny day, it generates heat for the cabin during the day and for a few hours after the sun sets. On cloudy days and for part of the night after a sunny day, the cabin would need another heat source. We’ll assume:

Our goal is a low thermal mass sunspace (LTMS) that delivers most of the heat it gains to the cabin. So during a sunny day, the sunspace is comfortable to be in, but most of the incoming solar heat is routed to the cabin. At night, the sunspace cools off and is not suitable for plants or people.

The sunspace has 130 square feet of south-facing and steeply tilted (or vertical) double glazing to collect solar heat. There are 50 square feet of R-2 windows in the cabin.

The sunspace has a set of vents and fans to efficiently deliver heat to the house. It has a way to effectively close these vents at night to reduce heat loss from the cabin to the sunspace.

The nonglazed surfaces of the sunspace—walls, ceiling, and floor—are well-insulated and have low thermal mass. The insulation slows heat loss to the outdoors, and the low-thermal-mass materials mean that the sunspace won’t be absorbing the heat you want to go to the cabin.

The cabin is sealed well enough to have an infiltration rate of 0.5 air changes per hour or less.

You’ll need to match the cabin’s heat loss to the sunspace’s heat gain. Looking at a sunny, 20°F day, the cabin’s heat loss (assuming the above construction details) comes out about 4,700 Btu per hour. (You can use the heat-loss calculator at bit.ly/BIS-HeatLossCalc.) Since you’re using efficient lighting and appliances, with a couple of people included, the internal heat gains might be 400 Btu per hour. That would give a net heat loss of 4,300 Btu per hour.

If the average outdoor temperature for the day is 20°F, then the 24-hour heat loss would be 103,200 Btu per day (4,300 Btu/hr. × 24 hrs.). This is about equivalent to 1.4 gallons of propane burned in an 80% efficient furnace.

That’s the simplified heat-loss calculation—you could be more accurate with a simulation that takes into account hourly temperature variations and other things. You could also do it for your actual weather for the whole heating season, day by day.

Our example sunspace solar heat gain looks like this: The solar insolation for a sunny winter day is about 300 Btu per square-foot•hour. If you assume that a LTMS can achieve 50% efficiency, then its solar heat output would be 19,500 Btu per hour (300 Btu/ft.2•hr. × 130 ft.2 × 0.5 efficiency).

This is more than four times the per-hour heat loss from the cabin. That’s good because:

On a sunny day, the heat output in the early morning and late afternoon will fall below 19,500 Btu per hour because of the incidence angle between the sun and the glazing. For instance, a 45° incidence angle would cut it by 0.707 (cosine 45°). So, you don’t get 19,500 BTU/hr. gain for every hour of the sunny day.

On partly sunny days and on cold days when the temperature is less than 20°F, the sunspace will still be able to provide a decent amount of usable heat.

Heat gain that is well above the cabin’s heat loss should help raise the temperature of the thermal mass above normal room temperature, allowing it to “store” heat for radiation back into the space as it cools.

The “extra” heat could be routed to a heat storage system to heat the cabin through the night.

It gives you more freedom to optimize the layout of the sunspace for the activities you want to do there, even if that means giving up some solar gain.

You’ll also need to follow the other design rules that are covered in the article “Low Thermal Mass Sunspaces” in HP158 and at the link above. Of particular importance is summer venting—the sunspace will be an oven without it.

The simple building energy performance estimators that I use are easy to understand, but if you want to take it to the next level of simulation accuracy, you might want to check out eQUEST or EnergyPlus, which also offers a plug-in for Google SketchUp.

Both of these software packages are free, but both have fairly steep learning curves. Don’t get so wrapped up in these simulations that you lose sight of the basic physics that are going on.

Comments (6)

>The sunspace needs to collect 24h(70-31.9)89-38K = 43.4K Btu on a sunny day and store another 43.4K Btu at 140 F for cloudy days... 1040Ad = 6h(80-35.6)Ad/R2 + 43.4K makes Ad = 48 ft^2 of daily heat glazing with 80 F air inside, and 1040Ac = 6h(160-35.6)Ac/R2 + 43.4K makes Ac = 48 ft^2 of storage glazing with 160 F air inside, for a total 113 ft^2 of sunspace glazing, which can be timeshared with an air-water heat exchanger that also provides heat from the tank on cloudy days.

We can model a constant 1040/6h = 173 Btu/h Sun for 6 hours on 113 ft^2 of timeshared glazing with a 35.6 F outdoor temp like this, viewed in a fixed font:

This seems like a better approach, since another 127-113 = 14 ft^2 sunspace glazing is cheap at $1.30/ft^2, compared to another pump and tank, especially since twinwall comes in 4'x8' sheets, with a choice between 113/32 = 3.5 and 127/32 = 3.97 sheets.

NREL Blue Book weather data indicate that December is the worst-case month for solar house heating in Columbus, OH, when 650 Btu/ft^2 falls on south walls on an average 31.9 F day with a 39.2 high and an average 35.6 daytime temp.

Gary Reysa writes:

>Our [less ambitious] goal is a low thermal mass sunspace that delivers most of the heat it gains to the cabin. So during a sunny day, the sunspace is comfortable to be in, but most of the incoming solar heat is routed to the cabin. At night, the sunspace cools off and is not suitable for plants or people.

A more ambitious goal would be enough heat storage for 4 cloudy days in a row, with an approximate 97% solar heating fraction and a sunspace that's above freezing at night. Making it comfortable for people during the day and also storing some higher temperature heat for cloudy days is difficult. A dark mesh curtain near the glazing could confine higher temp air to the gap between the glazing and the curtain, which could leave 70 F air in the bulk of the sunspace, with desirable privacy.

>The sunspace has 130 square feet of south-facing and steeply tilted (or vertical) double glazing to collect solar heat. There are 50 square feet of R-2 windows in the cabin. The cabin is sealed well enough to have an infiltration rate of 0.5 air changes per hour or less.

An ambitious building could have R2 south sunspace twinwall polycarbonate glazing with 80% solar transmission and 16 ft^2 of R4 windows and 0.2 ACH. With equally-likely 2x650 = 1300 Btu/ft^2 sunny and 0 Btu cloudy days and a 650 Btu/ft^2 average, the glazing would transmit 0.8x1300 = 1040 Btu/ft^2 on a sunny day.

With a 16ft^2/R4 = 4 Btu/h-F window conductance + 624ft^2/Rv walls + 512ft^2/Rv roof and floor + 0.2x2560ft^3/60 = 8.5 Btu/h-F for air leaks, the total building conductance G = 12.5+1149/Rv Btu/h-F. At 70 F, the building needs 24h(70-31.9)G = 11.4K+1051K/Rv Btu of heat on an average December day. A frugal 300 kWh/mo indoor electrical use and 2 300 Btu/h half-time occupants could contribute 38K Btu/day of that, leaving a need for 1051K/Rv-26.3K Btu/day of additional heat.

A mobile building could use a 4'x4'x3' tall plywood tank with a 10'x10' folded EPDM liner as a heat store, filled on site with 4x4x3x62.33 = 2992 pounds of water cooling from 140 F to 80 after 4 cloudy days... 4(1051K/Rv-26.3K) = (140-80)2992 Btu makes Rv = R15, eg 3" of Styrofoam, with G = 89 Btu/h-F.

The sunspace needs to collect 24h(70-31.9)89-38K = 43.4K Btu on a sunny day and store another 43.4K Btu at 140 F for cloudy days... 1040Ad = 6h(80-35.6)Ad/R2 + 43.4K makes Ad = 48 ft^2 of daily heat glazing with 80 F air inside, and 1040Ac = 6h(160-35.6)Ac/R2 + 43.4K makes Ac = 48 ft^2 of storage glazing with 160 F air inside, for a total 113 ft^3 of sunspace glazing, which can be timeshared with an air-water heat exchanger that also provides heat from the tank on cloudy days, with 2 motorized dampers to select sunspace or building air for the heat exchanger inlet and outlet.

My solarium is not only used for space heating, but is where we dry clothes on three available lines, grow food all winter and have year around herbs.

Lettuce, spinach, chard, tomatoes, marijuana (we live in Colorado) are grown all winter long. People need to know that food prices will escalate as climate change makes industrial agriculture (look at what is happening in the Central Valley of California, where drought is already impacting winter produce prices) more challenging with drought, drying and heat waves affecting output. If people do not want or cannot pay higher food prices, then they need to plan for a future where they grow their own food outside in the summer, preserving what they can for the winter. They can also have fresh greens and other veggies all winter long by using an existing sun space or planning and building a sun space on their existing home. I couldn't live without mine, it simply provides too many benefits besides space heating.

Awesome point, Steve. Just out of interest, how big is your sunspace/greenhouse? I'm planning a project here at my place, but I'm not a gardener. My sweetie is, but has no greenhouse experience yet. We're trying to figure out how big to go, without being excessive, nor wishing we had gone bigger in the future. Current plans are 10' by 16', including a potting bench and utility sink. Will we wish for more space too soon?

My sunspace is 44' long and varies from 4' to 6' wide and is part of our solar envelope home. At times I almost do not have enough front row space in the sunshine during the winter. Because my solarium has vertical glazing and a 16" roof over hang, the space is fully shaded at the summer solstice, and is not a good summer growing space. For the spring, summer and fall I have a greenhouse and outdoor gardens. But, it is very important to shade a sunspace attached to a house so you do not overheat the living area during the warm months. It is also wise to have opening from the living space to the sunspace close-able and to add ventilation options for warm days when there strong solar gain. Many times in September, our Colorado sunspace must be ventilated.

I am going to build a zero-energy straw-bale home this summer. The sunspace is going to be 6' x 24'. I will have to find new
homes for the large potted cacti that now populate our sunspace when we move into the new house with it's multi-functional sunspace.